The Clean Air Act Actually Caused More Rain to Fall on Atlanta

Among the many problems caused by pollution – and there are a lot of them – particulate matter in the air also messes with the weather. Pollution can suppress rainfall, meaning, in theory, that if we cut down on emissions from cars and factories, nearby areas might receive more rain.

Exactly this appears to have happened in Atlanta since the passage the Clean Air Act of 1970, and evidence collected there by Georgia State University researcher Jeremy Diem suggests that something very similar may have happened in other urban areas, too. Diem's research, published in the journal Atmospheric Environment, analyzed data collected between 1948 and 2009 at nine weather stations located around the Atlanta region, some as far as 80 kilometers away from the city center.

Throughout the 1950s, '60s and early '70s, summertime rainfall had decreased in the Atlanta region and downwind of it, where pollution blows in the summer months. But between 1970 and 1975, as a result of the Clean Air Act, particulate matter in the region dropped by about 40 percent. And summer rainfall rapidly bounced back, primarily in the form of more days with heavy rain.

"We went from having less rainfall than you’d expect to having more rainfall than you’d expect really abruptly," Diem says.

And what changed? The 1970 Clean Air Act (an update to the original 1963 legislation) authorized states and the federal government to draft powerful regulation limiting emissions from both industry and moving sources like cars. Diem argues that the law's passage was the only plausible explanation for this shift in precipitation in Atlanta, even taking into account that the urban heat island effect is thought by scientists to produce more rain than typically falls in rural areas.

Atlanta likely isn't the only American city that experienced this. Conversely, however, this research suggests that many industrializing metros outside of the U.S. may still be in the phase Atlanta experienced in the 1950s.

"I think suppression is probably occurring in a lot of places across the globe right now," he says. "Think of the Chinese cities, the places where there is just a lot of particulate matter."

About the Author

Emily Badger is a former staff writer at CityLab. Her work has previously appeared in Pacific Standard, GOOD, The Christian Science Monitor, and The New York Times. She lives in the Washington, D.C. area.